1. Field of the Invention
The present invention relates to a method for producing a ceramic body such as a ceramic substrate, in particular, to a method for producing a ceramic body by a non-contraction process.
2. Description of the Related Art
In recent years, there have been ever increasing demands for reduction in size, achievement of higher accuracy, and achievement of lower cost in electronic equipment. With such demands, achievement of higher accuracy and the like have also been demanded for ceramic bodies such as ceramic substrates used for circuit boards and the like. Herein, ceramic bodies include ceramic bodies including conductors and ceramic bodies not including any conductors.
In general, a ceramic body is produced by a step of preparing a ceramic material, a step of forming ceramic green sheets, a step of laminating the ceramic green sheets, and a step of firing the resultant laminated body. The laminated body contracts in the X-Y direction (hereinafter, referred to as “plane direction”) and the Z direction (hereinafter, referred to as “thickness direction”) in the firing step. Variation in the contraction in the plane direction in the firing step can cause problems such as degradation in positional accuracy of external conductor patterns, which considerably affects the quality of products.
To solve such problems, the non-contraction process in which a laminated body is allowed to contract only in the thickness direction while contraction in the plane direction is suppressed is proposed in, for example, Japanese Patent No. 2554415. Specifically, referring to FIG. 12, a ceramic laminated body is produced by respectively press-bonding constraining layers 48a and 48b that constrain contraction of a base layer 40 and contain a ceramic material to one and the other main surfaces of the base layer 40, which is obtained by laminating, as necessary, ceramic green sheets that can be fired at low temperature and is to be turned into a ceramic body after firing. After that, the ceramic laminated body is fired at low temperature. After the firing, the constraining layers 48a and 48b are removed by a technique such as a supersonic oscillation technique.
According to the method described in Japanese Patent No. 2554415, since the constraining layers 48a and 48b are constituted by ceramic green sheets that are not sintered at a temperature at which the base layer 40 can be sintered, the constraining layers 48a and 48b do not substantially contract in a firing step conducted at the temperature at which the base layer 40 can be sintered. Thus, contraction of the base layer 40 in the plane direction can be suppressed and the dimensional accuracy of the resultant ceramic body can be considerably enhanced.
In recent years, among ceramic bodies, with improvement in functions of ceramic bodies and the like (hereinafter, referred to as “ceramic substrates”) that include conductors for wiring or the like, there is an increasing demand for providing wiring of such ceramic substrates at higher density. Additionally, there is also a demand for embedding circuit elements such as capacitors in ceramic substrates. As a result, there is a case where the formation density of conductors or the degrees of contraction of conductors are different between one side and the other side with respect to a central plane in a ceramic substrate, the central plane being a plane in which the distance from one main surface of the ceramic substrate is equal to the distance from the other main surface of the ceramic substrate. In this case, the ceramic substrate suffers from warpage, deformation, and the like when being fired, which is problematic.
To deal with this problem, for example, Japanese Unexamined Patent Application Publication No. 2001-60767 proposes a method of suppressing warpage, deformation, and other defects of a ceramic substrate in a firing step by making the thicknesses of constraining layers provided on the main surfaces of the ceramic substrate different from each other. According to this method, contraction of a ceramic substrate in the plane direction can be prevented while warpage, deformation, and other defects of the ceramic substrate can be also prevented.
In this manner, in the non-contraction process, by utilizing the thicknesses of the constraining layers, a constraining force in the plane direction to prevent warpage, deformation, and other defects of the ceramic substrate in a firing step is improved.
However, the above-described method utilizing the thicknesses of constraining layers does not solve a problem with respect to the removal of constraining layers. Specifically, a the thicknesses of constraining layers increase, the amount of the constraining layers to be removed by a technique such as the above-described supersonic oscillation technique is increased, and a greater amount of time is required to remove the constraining layers.